Stem cells have the ability to divide for indefinite periods in culture and to become a wide variety of specialized cell and tissue types, which can then be used for basic research, drug discovery, and treatment (or prevention) of many diseases. Stem cells are typically divided into two main groups: adult stem cells and embryonic stem cells.
Adult stem cells are undifferentiated but are present in differentiated tissues, and are capable of differentiation into the cell types from which tissue the adult stem cell originated. Adult stem cells have been derived from various sources, such as the nervous system, bone marrow; adipose tissue, dermis, pancreas and liver. Stem cells have also been isolated from umbilical cord and placenta. It is believed that stem cells of the adult type are also found in smooth muscle tissue, striated muscle tissue, cardiac muscle tissue, bone tissue, bone spongy tissue, cartilage tissue, pancreatic ductal tissue, spleen tissue, thymus tissue, tonsil tissue, Peyer's patch tissue, lymph nodes tissue, thyroid tissue, epidermis tissue, dermis tissue, subcutaneous tissue, heart tissue, lung tissue, vascular tissue, endothelial tissue, blood cells, bladder tissue, kidney tissue, digestive tract tissue, esophagus tissue, stomach tissue, small intestine tissue, large intestine tissue, adipose tissue, uterus tissue, eye tissue (including retinal tissue), lung tissue, testicular tissue, ovarian tissue, prostate tissue, connective tissue, endocrine tissue, and mesentery tissue.
Embryonic stem cells are undifferentiated cells derived from the embryo. Typically, these cells are extracted from the inner cell mass of a blastocyte and when cultured under the unique conditions, either alone or in combination with a variety of feeder cells, the embryonic stem cells maintain euploid karyotype, do not undergo senescence, and retain the ability to differentiate into cells of the endodermal, ectodermal, and mesodermal lineages.
Mesenchymal stem cells (MSCs) refer to cells that have the potential to self-renew and are capable of differentiating into multiple mesenchymal lineages. These cells may have the capacity to give rise to other germ layer cell types such as neuronal cells. The major source of MSCs is isolation from bone marrow and other tissues, such as adipose tissue. Due to their scarcity in adult tissues, MSCs are normally isolated only in small numbers, however, they have an extensive capacity for proliferation and can be readily expanded in culture to generate clinically-relevant numbers of cells through multiple passages. These cells have been shown to support tissue repair and regeneration, making them a promising tool for various therapeutic applications.
One of the earliest clinical uses of stem cells was for performing bone marrow transplants in patients with hematological malignancies in which hematopoietic stem cells derived from the donor bone marrow were administered into the recipient subsequent to treating the recipient with a dose of radiation and/or chemotherapy in order to ablate not only the hematological malignancy but also non-malignant hematopoiesis. The administration of non-malignant hematopoietic stem cells results in donor-specific hematopoiesis, and in some patients, cure of the malignancy. This was first described in 1957 in patients with acute leukemia following myeloablation. Stem cells have also been utilized as an autologous bone marrow transplant in patients administered high doses of chemotherapy and/or radiation therapy for the treatment of solid tumors, in order to restore bone marrow. The use of autologous hematopoietic cell transplants combined with high dose chemo/radiotherapy for solid tumors has been extensively investigated for breast, colon, lung, nasopharyngeal cancer, and other types of cancers.
The clinical use of autologous stem cells has also been performed for a variety of autoimmune indications, including rheumatoid arthritis, multiple sclerosis, systemic lupus erythromatosis, and systemic sclerosis.
The importance of technologies utilized to expand stem cells, both of adult and/or embryonic derivation is illustrated by the clinical uses of these cells in treatment of a wide range of diseases. The cell culture systems used in these treatments (e.g., liquid static culture, semi-solid culture and long term bone marrow culture) appears to have their own unique requirements which must be met before one can culture stem cells of human or other mammalian species. To date, however, a common requirement and disadvantage of stem cell culture systems has been the requirement for undefined components contained in animal sera (e.g., fetal bovine serum or horse serum) for optimal growth or expansion of stem cells.
The use of serum in the culture of hematopoietic cells is disadvantageous for several reasons. Serum is a major source of undefined differentiation factors and thus tends to promote hematopoietic cell differentiation, rather than expansion. The efficiency of serum varies between lots of serum. Some lots of serum have been found to be toxic to cells. Moreover, serum may be contaminated with infectious agents such as mycoplasma, bacteriophage, and viruses. These problems cause inconsistencies in the growth-supporting properties of the medium, making standardization of stem cell production processes difficult and making the interpretation of experiments carried out in serum-containing media difficult. Thus, the use of serum represents a major obstacle for the clinical implementation of stem cell-related therapies.
As a result, researchers have attempted to replace animal sera or conditioned media with serum-free culture media of varying degrees of chemical definition. These attempts have met with varying degrees of success, depending upon the identity of the cell type one is trying to expand. The development of serum-free media has been reviewed (Sandstrom, E. E. et al., Biotech. & Bioengin. 43:706-733 (1994); Collins, P. C. et al., Curr. Opin. Biotech. 7:223-230 (1996); McAdams, T. A. et al., TIBTECH 14:341-349 (1996)). A more attractive alternative would be a defined serum-free medium. Therefore, it is desirable to develop defined serum-free media and methods for (i) isolating stem cells and (ii) expanding these cells for an extended period of time through multiple passages while maintaining their multi-lineage differentiation potential. The creation of defined serum-free media and methods should provide a robust platform that will help to enable the clinical implementation of stem cell-based therapies.